Back to basics with SAW filters
01 November 2007
In today’s crowded RF spectrum, the ability of a wireless design to control signal bandwidth is essential both for performance and for regulatory acceptance
Controlling bandwidth is the job of filters and one of the most cost-effective and stable RF filter technologies is not electronic but mechanical. Surface acoustic wave (SAW) filter technology provides a host of benefits in wireless system design, and understanding SAW basics is the first step to applying the technology effectively.
When a crystalline material having some elasticity receives a mechanical shock, a wave moves across its surface. With a piezoelectric crystal, an electrical signal applied via a transducer can induce such a surface wave. The wave, in turn, can also induce an electrical signal into the transducer.
SAW devices take advantage of these effects to manipulate the signal while it is in mechanical form. One of the simplest uses of SAW technology is in the creation of pass band filters. A SAW filter essentially comprises a slice of piezoelectric material with interleaved metal fingers deposited on the surface forming interdigital transducers (IDT) at both ends. The IDT finger spacing is set to support resonance of the surface wave at desired frequencies while dampening or deflecting wave energy at other frequencies, providing a filtering effect.
SAW filter factors The detailed characteristics of a SAW filter depend on many factors, including type of crystal material, size of slice, and the thickness and placement of the IDT fingers. In general, however, SAW filters can be constructed that operate anywhere in the 20MHz to 2.6GHz frequency range with a pass band from 0.1 per cent to 60 per cent of centre frequency.
These factors make SAW filters suitable for applications involving RF and microwave signal filtering, both at fundamental and intermediate frequencies.
As a filter, SAW devices have many desirable characteristics. One of the most important is selectivity; SAW filters typically exhibit a sharp roll off outside the pass band. A representative filter with a 770MHz centre frequency and 13.5MHz bandwidth, for example, has sharp roll off.
Almost as important, SAW filters are small, about 140mm2, and passive devices that are easy to design with. They are simply inserted into the signal path; they require no external components other than for impedance matching, and use no power. Thus, they only require four active pins, two input and two output, and can be housed in extremely small packages.
SAW filters are also inherently stable. As they have no dependence on external electronic components there are no concerns for component drift. The filter’s thermal characteristics depend solely on the expansion coefficient of the crystal slice. For materials such as quartz, first-order thermal drift is essentially zero and for other materials less than 20 parts per million per ºC.
Another desirable characteristic is that SAW filters exhibit a group delay that is essentially flat within the pass band, providing a linear phase response. This characteristic arises from the signal’s mechanical representation during filtering. The surface wave does not undergo significant dispersion, so all the mechanical energy propagates through the filter at essentially the same speed. This means that SAW filters do not introduce phase distortions, making them suitable for use with the complex phase, and frequency, modulation schemes of today’s wireless technologies.
Mechanics limits power
The mechanical nature of SAW filters does have its limitations, however. One is its power handling capability. The signal strength that a SAW filter can tolerate depends on the height of the wave it can safely propagate. The characteristics of the typical crystal materials in use limit the input signal strength to around 31dBm.
Mechanics is also responsible for restrictions on the total out-of-band rejection that a SAW filter can achieve. Dampening of the undesired frequencies depends on the number of metal fingers that lie in the path; each deflecting a fraction of the wave energy it intercepts. The more fingers, the greater the rejection, but with diminishing returns. A single filter slice can achieve no more than 55dB to 60dB. To achieve greater rejection, two or more filters must be cascaded.
As they are passive devices, however, SAW filters in cascade will exhibit additional signal loss. Each filter device has an insertion loss, typically 2dB to 3dB. These losses add together as filters cascade. Thus, there is a trade-off between rejection ratio and insertion loss that users must make.
There are other tradeoffs in SAW filter design, as well. Many of these characteristics depend on the crystal material being used along with the size, thickness, and composition of the transducer finger material. Fortunately for users, a detailed understanding of the physics is not required. Some vendors offer customised SAW filter designs. For example, Integrated Device Technololgies (IDT) can produce simulations for customers to review when deciding on such trade-offs.
Developers who want to use SAW filters, then, need only determine their design requirements and approach the vendor. While there are many details to determine, the key parameters to know are centre frequency, bandwidth, and packaging constraints as well as insertion loss and rejection requirements. Armed with these, vendors can help direct designers to the most appropriate standard product or initiate discussions for creation of a custom product.
The many benefits of SAW filters have earned them a place in a variety of applications. Their simplicity and small size make them suitable for wireless and handheld devices. Their low cost allows them to be used in a wide variety of other consumer devices, as well, including keyless entry. Their performance makes them capable of handling demanding applications such as military systems and the tightly-regulated cellular telephony market. Developers who understand where and how to apply SAW filters will have an advantage in solving tough RF design challenges.
STEPHEN F. MAGNO is SAW products marketing manager; BOB HOLLOWAY is senior product support engineer, IDT MicroNetworks Division
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